Drive control for an electric drive with a secure electrical separation of power element and control element

ABSTRACT

The invention relates to a drive control for an electric drive with a secure electrical separation of power element and control element. The aim of the invention is to reduce the number of components such as optic couplers and buffer amplifiers between the power element and a control electronics. To this end, a suitable electrical transformer (U) is inserted in a digital communication interface (K) between the control unit (R) and the control electronics (A) for the purpose of providing a secure electrical separation. To make use of a transformer (U) possible, a non-zero frequency encoding, for example a Manchester encoding, is carried out. Alternatively, an Ethernet physics can be used to provide a suitable communication interface. The transformer electrically insulates the two communication paths from each other that are provided in an Ethernet physics and preferably has little coupling capacity and a low attenuation factor.

BACKGROUND OF THE INVENTION

The invention relates to a drive control system for an electrical drivehaving a power section which is at a comparatively high electricalpotential, having drive electronics which supply the power section withdrive signals, and which receives phase current actual values from thispower section, and having a control unit which is at a comparatively lowelectrical potential and is connected to the drive electronics via adigital communications interface, with DC isolation between the powersection and the control unit.

For safety reasons, the high-voltage side of the power section (up to720 volts) must be safely electrically isolated from the low-voltageside of the control unit (generally approximately 5 volts) in a drivecontrol system such as this. DC isolation is required at some point forthis purpose, with the relevant Standards requiring an air gap andcreepage distance of, for example, 8 mm. To achieve this, the electricalcomponents which are used must comply with the requirements in thesespecific Standards.

Present-day solutions for such safe electrical isolation provide thisisolation between drive electronics and the power section. A knownarrangement of a drive control system such as this is illustrated in theform of a block diagram in FIG. 3.

The power section LT draws its power from an intermediate circuit ZK ofa converter (not shown), and provides the three phase voltages fordriving a motor M. The drive electronics A produce six drive signals Uxvia six respective optocouplers OK (for the sake of clarity, only onesuch optocoupler is shown by way of example), which are used for drivingcurrent valves which are arranged in the power section, in particular abridge circuit for IGBT transistors. In addition, a further path Ubrsuch as this may be required for driving a braking chopper.

The respective phase current actual values lactR, lactS, lactT in thethree phases R, S, T of the motor M are measured, and are passed viarespective isolating amplifiers TV to the drive electronics. Inindividual cases, only two isolating amplifiers are also provided, sincethe current in the third phase can be obtained from the two detectedphase current actual values. In addition, an isolating amplifier TV isrequired for detecting the intermediate-circuit voltage Uzk. The driveelectronics are connected to the control unit R via a communicationinterface K.

Thus, according to the prior art, the six or seven (with the brakingchopper) optocouplers OK and the isolating amplifiers TV must complywith the requirements for safe electrical isolation. Components such asthese are comparatively expensive, as a result of which the fact that alarge number of these components that have been mentioned are requiredmeans that this approach is highly costly.

SUMMARY OF THE INVENTION

The object of the present invention is thus to provide a simpler andhence lower-cost drive control system for an electrical drive with safeelectrical isolation between the power section and the control unit.

According to the present invention, this object is achieved bydeveloping a drive control system of the type mentioned initially suchthat the DC isolation is provided solely by connecting a suitableelectrical transformer in the communication interface between thecontrol unit and the drive electronics.

The transformer is in this case preferably designed such that the DCisolation which is ensured by the electrical transformer complies withthe requirements for safe electrical isolation.

It is often also necessary to allow bidirectional transmission. Toachieve this, a solution is proposed with components based on theEthernet Standard, as is widely used for office communication, with thedigital communication interface being designed on the basis of Ethernetphysics, with the transformer providing DC isolation for both of thecommunication paths which are provided using Ethernet physics.

In this case, it has been found to be advantageous for the transformerto be integrated in the drive electronics.

Ethernet therefore does not allow bidirectional transmission to beprovided via one channel, and two separate channels are required,associated with increased complexity. A further restriction is that thetransformer for Ethernet does not, as standard, ensure adequateinterference immunity against rapid voltage rates of change (du/dt), inparticular does not ensure safe isolation for voltages above 720 V. Forthis reason, a suitable transformer which complies with theserequirements is provided as an extra item according to the invention.

A further advantageous refinement of the drive control system for anelectrical drive according to the invention is distinguished in that acoder is provided for coding a binary data stream without any DCcomponent at the transmission end, by which means the coded data streamcan be transmitted via the digital communication interface to theelectrical transformer and having a decoder for decoding the data streamat the reception end.

This allows binary information to be transmitted between the controlunit and the drive electronics, with the occurrence of DC componentsbeing avoided in the data stream via the communication interface, whichthe transformer would not be able to process.

In this case, it has been found to be advantageous for the coder at thetransmission end to be able to produce synthetic signal changes bytransmitting binary values of the data stream as a defined sequence ofsignal changes, and for the decoder to be able to recover the originalbinary values of the data stream by associating the associated binaryvalues with the sequences of signal changes arriving at the receptionend.

It is particularly advantageous in this case for the coder and decoderto carry out Manchester coding, since a large number of low-coststandard components are available for this purpose.

A serial communication interface for transmission can be provided by theparticularly low level of complexity on the basis of the invention, inwhich case transmission can be carried out using the half-duplex mode.

In order to allow synchronous digital communication between the controlunit and the drive electronics with a data signal and a clock signal, ithas been found to be advantageous for the coder to be used for logicallylinking the data signal and clock signal at the transmission end suchthat a coded data signal without any DC component results, which can betransmitted via a first communication path through the communicationinterface. A second communication path through the communicationinterface is then used for transmitting the clock signal, and theelectrical transformer is designed such that it provides DC isolationfor both communication paths, in which case the decoder can recover theoriginal data signal at the reception end by once again logicallylinking the coded data signal and the clock signal.

In order to avoid delay time differences between the data signal and theclock signal, it is recommended that the means for coding and decodingthe clock signal use the same logical linking with a constant binaryvalue, and that it be possible to transmit the coded clock signal viathe second communication path.

This can be achieved particularly simply and hence at low cost if thecoder is used for exclusive-OR linking of the data signal and clocksignal at the transmission end, and the decoder is used for renewedexclusive-OR linking of the coded data signal and clock signal at thereception end.

According to the invention, for the clock signal and in order to avoiddifferent delay times, the coder can then be used for exclusive-ORlinking of the clock signal with a constant binary value, in particularwith the value zero, at the transmission end, in which case the clocksignal coded in this way can be transmitted via the second communicationpath, and in which case the decoder can carry out renewed exclusive-ORlinking of the coded clock signal and of the same constant binary value,in particular with the value zero, at the reception end.

It is to be found to be advantageous for all these measures for atransformer to be provided which has a low coupling capacitance betweenits primary circuit and its secondary circuit, in particular having acoupling capacitance of less than 1 pF.

Furthermore, a low-damping transformer should advantageously beprovided, in particular in order to process high transmission data rateswhich can be achieved on the basis of Ethernet physics.

Further advantages and details of the present invention will becomeevident from the exemplary embodiment described in the following textand in conjunction with the figures. In this case, elements having thesame functionality are identified by the same reference symbols. In thefigures:

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 shows a block diagram of a drive control system with simple andsafe electrical isolation according to the invention,

FIG. 2 shows a block diagram of a drive control system as shown in theFIG., based on Ethernet physics, and

FIG. 3 shows a block diagram of a drive control system with safeelectrical isolation based on the prior art.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

The illustration in FIG. 1 shows a block diagram of a drive controlsystem with simple and safe electrical isolation according to theinvention. The arrangement corresponds essentially to that according tothe prior art as shown in FIG. 3, as has already been described in theintroduction. The critical difference according to the invention is thatthe communication interface K has a transformer U, which provides thesafe electrical isolation SET.

The communication methods explained in the following text allow DCisolation of the data lines in the communication interface K, and thusmake it possible to move the safe electrical isolation SET to thecommunication interface K.

This means that there is no need for the number of expensiveoptocouplers OK and isolating amplifiers TV as are required according tothe prior art in FIG. 3. Since only functional isolation is now requiredbetween the high-voltage side and the low-voltage side, an air gap andcreepage distance of only 4 mm, for example, is now required. This makesit possible to achieve considerable cost savings.

According to the invention, the data stream is first of all preprocessedin a suitable manner and is then transmitted via the transformer U. Thistransformer should have a very low coupling capacitance between theprimary and secondary sides (typically less than 1 pF), in order toavoid problems with high voltage rates of change du/dt. Furthermore,this transformer should have low damping, in order to allow high datarates to be transmitted and in order to be suitable for safe isolation,SET.

One possible way to communicate via a communication interface K which isprotected by such a transformer U is to use communication componentsbased on the widely used Ethernet Standard, with the digitalcommunication interface being designed on the basis of Ethernet physics,and with the transformer providing DC isolation for both communicationpaths which are provided using Ethernet physics.

A detail of one such option is shown in the illustration in FIG. 2. Forthis purpose, the drive electronics A and the control unit R have linedrivers PL based on the Ethernet physical layer, which are operated witha suitable transmission protocol. The actual transmission takes placevia the transformer U, which provides DC isolation for bothcommunication paths.

Since, as already mentioned further above, Ethernet does not allowbidirectional transmission via one channel, two separate channels RX1and TX1 are needed. A transformer for Ethernet which, as standard, doesnot ensure sufficient interference immunity against high voltage ratesof change (du/dt), but which in particular does not ensure safeisolation for voltages above 720 V, is thus replaced by a transformeraccording to the invention. This transformer is designed such that itensures safe DC isolation for both communication paths or channels.

When using the described Ethernet physics, there is no problem with DCcomponents when transmitting binary data via the transformer since,according to the Ethernet Standard with the three voltage states thatare provided in this standard, with a positive voltage, a negativevoltage and zero volts, there is no need to be concerned aboutsaturation states in the transformer.

One example of a suitable synchronous transmission system is acommunication network based on Ethernet physics, which is strengthenedby means of a suitable digital transmission protocol to form adeterministic transmission system.

Since the application illustrated in FIG. 1 generally requires not onlyhigh-precision compliance with real-time conditions but also a highlevel of safety in the transmission process, the standardizedtransmission layer 2 (message frames and access methods) for (fast)Ethernet, which does not comply with these requirements, is completelyredefined, by way of example, by a new data protocol and a new accesscontrol system, and the Ethernet physics is thus used as the basis forreal-time communication between, for example, drive components. Thecommunication between the control unit R and the power section LT can beprovided in this way.

With reference to synchronization between a master, for example thecontrol unit R, and slave units, for example a number of power sectionsLT in a decentralized drive system, it has been found to be advantageousfor the slave units to be synchronized to the master unit by using apredetermined overall cycle time via a respective timer as a clock foreach slave unit, which time is set cyclically by reception of arespective slave-specific synchronization information item which isdefined by the master unit.

It is thus possible to use a master-slave communication architecture. Inorder to make it possible to interchange data cyclically at the samesampling times, a common time base is produced for the master and forall the slaves. The slaves are synchronized to the master byspecifically transmitted messages, relating to defined times, from themaster to the slaves, and individually configured timers in the slaves.

In this case, payload data messages and specific synchronizationmessages which contain the respective synchronization information may betransmitted. Alternatively, the synchronization information may also beintegrated in a transmitted payload data message.

In this case, the robustness of the communication system can be furtherimproved if each timer in a slave unit automatically starts a new cycle,even in the absence of the respective synchronization information, oncea predetermined overall cycle time has elapsed.

By way of example, a timeslot access method, which is initialized by themaster in the network and allows dead-time-optimum data transmission, isused for the transmission and reception modes during cyclic datatransmission. It is thus possible to precisely monitor the messages fordisturbed, premature or delayed transmission.

For this purpose, only the master unit has transmission authorization onthe communication path for initialization, and informs each slave unit,which only has response authorization, via an appropriate slave-specificmessage not only of the overall cycle time but also of the timeslotswithin the overall cycle time in which the respective slave unit willreceive messages from the master unit, and in which timeslots it shouldsend its messages.

In this case, it has been found to be advantageous for each slave unitto be informed of the respective synchronization time, during theinitialization phase.

If instantaneous values (for example phase current actual values of aconnected motor M etc.) are in each case stored relating to a commontime, in particular relating to the start of a cycle, in each slaveunit, that is to say the respective power section LT with its driveelectronics A, it is possible to achieve simultaneous and equidistantsampling for the control unit R.

Furthermore, monitoring information which allows safety functions thatare provided directly in the slave unit to be activated can be providedin each message that is transmitted from the master unit to a slaveunit.

The payload data can be transported in a message frame which, inaddition to slave addressing and message length information, providesprotection of the data integrity by means, for example, of a CRCchecksum and further safety-related data areas. The data in the messageframe may be evaluated not only by an application processor, but also bya communication module COM.

For this purpose, each slave unit transmits a signal with each messageto the master unit. The master unit then stops the appropriate slaveunit in a controlled manner if this signal is absent.

Although the transmission technique based on the Ethernet Standardprinciple that is used allows only point-to-point connections, the useof network nodes (so-called hubs) as in (fast) Ethernet networks allowsthe formation of networks, with a number of communication subscribers oreach communication subscriber having a circuit part for forming networknodes, which is used for passing on messages in the direction of anothermaster unit or further slave units with communication betweencommunications subscribers via network nodes likewise taking place usingthe procedure described above. According to the invention, eachcommunication interface K is then safely electrically isolated from thehigh-voltage side of the power section LT by means of suitabletransformer U.

Real-time communication can be achieved on the basis of a communicationsystem using Ethernet physics with the aid of the procedure describedabove. In this case, hierarchical networks can also be produced, withpoint-to-point connections (connected via network nodes) using Ethernetphysics for carrying out real-time communication in relatively largenetwork topologies.

Communication networks other than those described by way of exampleabove may, of course, also be used to provide the safe electricalisolation SET between the power sections LT according to the inventionand to provide for the networking with a control unit R, provided thatthe bandwidth on the transmission ensures communication at the currentregulation clock rate.

In this case, it is important to note that coding without any DCcomponent is carried out first of all for use of a transformer even withother digital communication methods, with one possibility here being, byway of example, Manchester coding. This makes it possible to avoid whatamounts effectively to a DC voltage being applied to the transformer U(which cannot process this DC voltage) as a result of a sequence of anumber of identical binary values.

For this purpose, encoders/decoders EC_DC are in each case provided bothin the control unit R and in the drive electronics A, that is to say atboth ends of the communication path, in the drive control system shownin FIG. 1. This results in data being coded without any DC component,for example being Manchester-coded, and being available as coded data.

A large number of other possible codings can of course likewise be used,provided that they allow signal transmission without any DC component.

A further exemplary embodiment allows transmission of synchronous data,with the signals being suitably coded in order to avoid the productionof DC components, by EXOR-linking of the clock signal and data.

To do this, two data streams must be transmitted, namely a data signaland the clock signal which is required for synchronous transmission. Twocommunication paths are therefore provided, which are protected by onetransformer, in a similar manner to that shown in FIG. 2, with safe DCisolation.

Initially, the data signal may have a DC component. In order to allow itto be transmitted by a transformer despite this, it is first of allexclusive-OR or EXOR-linked to the clock signal. This results in a codeddata signal. The clock signal can be EXOR-linked with a constant binaryvalue such as “zero” in order to avoid different delay times.

The logical linking of the data signal and clock signal thus ensuresthat a coded data signal without any DC component can be transmitted viathe transformer U. The clock signal itself never has any DC components.

Both coded signals are then transmitted via the respective communicationpath, and the original data stream is recovered by once againEXOR-linking the two coded signals. In order to recover the originalclock signal, this is once again EXOR-linked to the constant binaryvalue.

The data signal and the clock signal may, of course, be logically linkedin other possible ways which allow the data signal to be transmittedwithout any DC component. The EXOR-linking as explained above isdistinguished by its particularly simple and effective implementationfor the invention.

1. A drive control system for an electrical drive, comprising: a powersection which is at a comparatively high electrical potential; a driveelectronics which supplies the power section with a plurality of drivesignals and receives a plurality of phase current actual values from thepower section; a digital communications interface; a control unit whichis at a comparatively low electrical potential and is operativelyconnected to the drive electronics via the communications interface; andan electric transformer, disposed in the communications interface, forimplementing a DC isolation between the comparatively high electricalpotential of the power section and the comparatively low electricalpotential of the control unit.
 2. The drive control system of claim 1,wherein the transformer is integrated in the drive electronics.
 3. Thedrive control system of claim 1, wherein the transformer is integratedin the control unit.
 4. The drive control system of claim 1, wherein theelectrical transformer is constructed to ensure a DC isolation whichcomplies with requirements for a safe electrical isolation.
 5. The drivecontrol system of claim 1, wherein the communications interface isdesigned on the basis of an Ethernet physics, with the electricaltransformer providing DC isolation for a plurality of communicationpaths which are provided using Ethernet physics.
 6. The drive controlsystem of claim 1, wherein the communications interface uses a serialcommunication interface for transmission.
 7. The drive control system ofclaim 1, wherein the communications interface uses a half-duplex modetransmission method.
 8. The drive control system of claim 1, and furthercomprising a coder provided for encoding a binary data stream to createa coded data stream without any DC component for transmission to theelectrical transformer, and a decoder receiving the data stream via theelectrical transformer for decoding the data stream.
 9. The drivecontrol system of claim 8, wherein the coder produces synthetic signalchanges through transmission of binary values of the data stream as adefined sequence of signal changes, and the decoder recovers Previouslypresented binary values of the data stream by associating pertainingbinary values with the sequences of signal changes which arrive at thedecoder.
 10. The drive control system of claim 8, wherein the coder andthe decoder use Manchester coding.
 11. The drive control system of claim8, wherein the digital communications interface is a synchronous digitalcommunications interface between the control unit and the driveelectronics using a data signal and a clock signal.
 12. The drivecontrol system of claim 11, wherein the communications interface has afirst communication path and a second communication path, wherein thecoder logically links the data signal and the clock signal as to providean encoded data signal without any DC component which is transmittablevia the first communication path, and wherein the clock signal istransmitted via the second communication path, said electricaltransformer being so designed as to provide DC isolation for bothcommunication paths, with the decoder configured for recovering the datasignal by logically linking the coded data signal and the clock signal.13. The drive control system of claim 12, and further comprising meansfor coding and decoding of the clock signal by using a same logicallinking with a constant binary value, wherein the coded clock signal istransmittable via the second communication path.
 14. The drive controlsystem of claim 12, wherein the coder is constructed for exclusive-ORlinking of the data signal and the clock signal when transmitting andwherein the decoder is constructed for deciphering a renewedexclusive-OR linking of the coded data signal and the clock signalduring reception.
 15. The drive control system of claim 14, wherein thecoder is constructed for exclusive-OR linking of the clock signal with aconstant binary value, when transmitting, with the thus coded clocksignal transmittable via the second communication path, and wherein thedecoder is constructed for deciphering a renewed exclusive-OR linking ofcoded data signals and same constant binary value during reception. 16.The drive control system of claim 15, wherein the binary value is zero.17. The drive control system of claim 1, wherein the electricaltransformer has a primary circuit and a secondary circuit, with a lowcoupling capacitance between the primary circuit and the secondarycircuit.
 18. The drive control system of claim 17, wherein the couplingcapacitance is less than 1 pF.
 19. The drive control system of claim 1,wherein the electrical transformer is constructed to have a low dampingfeature.
 20. The drive control system of claim 12, and furthercomprising means for coding and decoding of a common-time value storedby components of the drive control system to create a coded data streamfor transmission to the electrical transformer, and a decoder receivingthe data stream from the electrical transformer for decoding the datastream.
 21. The drive control system of claim 20, wherein thecommon-time values are time values set cyclically by the reception of asynchronization information data item.
 22. The drive control system ofclaim 20, wherein the instantaneous values are set initially and apredetermined cycle time is used to transmit data items.